The present invention is directed generally to the transmission of signals in optical communications systems. More particularly, the invention relates to optical assemblies and apparatuses for use in optical communications systems.
The development of digital technology provided the ability to store and process vast amounts of information. While this development greatly increased information processing capabilities, it was soon recognized that in order to make effective use of information resources it was necessary to interconnect and allow communication between information resources. Efficient access to information resources requires the continued development of information transmission systems to facilitate the sharing of information between resources. One effort to achieve higher transmission capacities has focused on the development of optical transmission systems. Optical transmission systems can provide high capacity, low cost, low error rate transmission of information over long distances.
The transmission of information over optical systems is typically performed by imparting the information in some manner onto an optical carrier by varying characteristics of the optical carrier. In most optical transmission systems, the information is imparted by using an information data stream to either directly or externally modulate an optical carrier so that the information is imparted at the carrier frequency or on one or more sidebands, with the later technique sometimes called upconversion or sub-carrier modulation (xe2x80x9cSCMxe2x80x9d).
SCM techniques, such as those described in U.S. Pat. Nos. 4,989,200, 5,432,632, and 5,596,436, generally produce a modulated optical signal in the form of two mirror image sidebands at wavelengths symmetrically disposed around the carrier wavelength. Generally, only one of the mirror images is required to carry the signal and the other image is a source of signal noise that also consumes wavelength bandwidth that would normally be available to carry information. Similarly, the carrier wavelength, which does not carry information in an SCM system, can be a source of noise that interferes with the subcarrier signal. Modified SCM techniques have been developed to eliminate one of the mirror images and the carrier wavelength, such as described in U.S. Pat. Nos. 5,101,450 and 5,301,058.
Initially, single wavelength carriers were spatially separated by placing each carrier on a different fiber to provide space division multiplexing (xe2x80x9cSDMxe2x80x9d) of the information in optical systems. As the demand for capacity grew, increasing numbers of information data streams were spaced in time, or time division multiplexed (xe2x80x9cTDMxe2x80x9d), on the single wavelength carrier in the SDM system as a means to better use the available bandwidth. The continued growth in demand has spawned the use of multiple wavelength carriers on a single fiber using wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d). In WDM systems, further increases in transmission capacity can be achieved not only by increasing the transmission rate of the information on each wavelength, but also by increasing the number of wavelengths, or channel count, in the system.
There are two general options for increasing the channel count in WDM systems. The first option is to widen the transmission bandwidth to add more channels at current channel spacings. The second option is to decrease the spacing between the channels to provide a greater number of channels within a given transmission bandwidth. The first option currently provides only limited benefit, because most optical systems use erbium doped fiber amplifiers (xe2x80x9cEDFAsxe2x80x9d) to amplify the optical signal during transmission. EDFAs have a limited bandwidth of operation and suffer from non-linear amplifier characteristics within the bandwidth. Difficulties with the second option include controlling optical sources that are closely spaced to prevent interference from wavelength drift and nonlinear interactions between the signals.
A further difficulty in WDM systems is that chromatic dispersion, which results from differences in the speed at which different wavelengths travel in optical fiber, can also degrade the optical signal. Chromatic dispersion is typically controlled using one or more of three techniques. One technique is to offset the dispersion of the different wavelengths in the transmission fiber through the use of optical components such as Bragg gratings or arrayed waveguides that vary the relative optical paths of the wavelengths. Another technique is to intersperse different types of fibers that have opposite dispersion characteristics to that of the transmission fiber. A third technique is to attempt to offset the dispersion by prechirping the frequency or modulating the phase of the carrier source in addition to modulating the data onto the carrier. For example, see U.S. Pat. Nos. 5,555,118, 5,778,128, 5,781,673 or 5,787,211. These techniques require that additional components be added to the system and/or the use of specialty optical fiber that has to be specifically tailored to each length of transmission fiber in the system.
New fiber designs have been developed that substantially reduce the chromatic dispersion of WDM signals during transmission in the 1550 nm wavelength range, such as dispersion shifted fiber and non-zero dispersion shifted fiber. However, the decreased dispersion of the optical signal allows for increased nonlinear interaction between channels, such as four wave mixing, which increases signal degradation. The effect of lower dispersion on nonlinear signal degradation becomes more pronounced at increased bit transmission rates due to the higher signal launch power at higher bit rates.
Non-linear interactions can be significantly reduced if the data signals are linearly polarized and oriented orthogonal to each other. For example, see U.S. Pat. No. 5,111,322, issued on May 5, 1992. Such systems, however, can be problematic. For example, the system taught in the ""322 Patent requires two modulators to produce a pair of orthogonal signals. As a result, the size, cost, and power consumption of such systems will increase significantly as the number of WDM channels increase. There is a need to reduce the number of components, particularly expensive components like modulators, while at the same time reducing the effects of phenomenon such as chromatic dispersion and non-linear interactions.
With the increase in the number of wavelength channels comes the need for more components to process and transmit the channels. Because space is often limited where this transmission equipment is deployed, there is a need for the equipment to be as compact as possible. This leads to optical assemblies with very closely spaced components. These optical assemblies will contain various components that may produce magnetic fields. Various optical components have optical properties that are sensitive to magnetic fields. External magnetic fields can degrade the performance of optical components in the optical assemblies. As a result, there is a clear need for systems and apparatuses to protect optical components from magnetic fields in compact optical assemblies and optical communication systems.
The present invention is directed to an optical assembly with a substrate. A non-latching optical component is attached to the substrate and includes a non-latching garnet and a permanent magnet in magnetic communication with the garnet. Also, a latching optical component is attached to the substrate and includes a latching garnet. A latching optical component as used herein means what is commonly known as a self-latching optical component. A magnetic shield is between the non-latching optical component and the latching optical component.
This and other embodiments of the present invention will be described in the following detailed description. The present invention addresses the needs described above in the description of the background of the invention by providing protection to closely spaced optical components in optical assemblies from external magnetic fields that might degrade the performance of the optical components. These advantages and others will become apparent from the following detailed description.